In rotary offset printing presses, the ink is supplied by means of ink metering elements on an ink duct cooperating with a ductor roller. In these presses a layer of ink of varying profile is produced on the ductor roller, as a result of the positioning of the ink metering elements relative to the ductor roller. To achieve optimum reproducibility of these layers of ink, it is essential that the ink duct should be mounted accurately relative to the ductor roller. It will also be understood that distortion occurs in the ink duct area and at the ductor roller due to the weight and high viscosity of the relatively thick printing ink. Nevertheless, the press operator must also be able to disengage the ink duct a sufficient distance from the ductor roller for cleaning and adjustment purposes to allow optimal operation.
To minimize distortion at the ductor roller and ink duct, it has been proposed, according to DE-OS 3,033,998, to mount the ink duct on two trunnions of the ductor roller between the two side frames. For this purpose, a bearer ring, for example in the form of a roller bearing, is provided on each roller trunnion and a mounting for securing the ink duct is disposed around it. Engagement is effected by means of a clamping element which connects the mounting (e.g., a steel band) to the ink duct. The ink duct is fixed with respect to the machine frame by means of retaining pins in enlarged bores and an elastic coupling between the ink duct and the retaining pins. In this way, when the ink duct is engaged against the ductor roller it can be slightly lifted out of its mountings, which are fixed on the machine frame.
The foregoing arrangement, however, is particularly disadvantageous inasmuch as the weight of the printing ink and ink duct have a direct effect on its suspension Also, when the ink duct is engaged with the ductor roller, additional forces from the elastic mounting at the retaining pin act on the ductor roller. Also, the ink duct can be swung away only at the defined pivot point of the retaining pins, i.e., very closely past the ductor roller. Therefore, operation is not very user-friendly, since the release and fixing of the clamping means is complex and accessibility thereto is obstructed.
Another arrangement of the ink metering system is shown in DE-PS 3,427,909. Here the ink duct is supported on the surface of the ductor roller by way of cam rollers. To ensure continuous contact between the cam rollers and the ductor roller, the ink duct is engaged with the ductor roller by means of a lever drive with the interposition of a relatively stiff spring set from the machine frame. In these conditions the ink duct is also lifted out of a pivot bearing fixed on the machine frame.
The disadvantage of this latter arrangement is that relatively large forces are required to enable the ink duct to be reliably engaged against the surface of the ductor roller. These forces must at least be such as to ensure that the ink duct is engaged with adequate forces even when fully charged with printing ink. These forces, however, act on the ductor at all times and naturally tend to bend it. Depending on the ink duct loading, the roller flexure varies, since at full duct loading there is minimal flexure, while when the ink duct is substantially empty there is maximum ductor roller flexure. The fact that the ink duct moves closely past the ductor roller also prevents the ink collecting space beneath the metering elements from being positioned so as to be readily cleaned and have a volume sufficient for long periods of operation.
Accordingly, the printing ink to be introduced into the ink duct and its effect due to weight and hydrodynamic forces, in conjunction with the manufacturing tolerances relative to the ductor roller, must be taken into account as further problems to overcome for exact positioning.